mmg_233_2013_genetics_genomicswikiaorg-20200214-history
S. pombe: Identification of cell division cycle (cdc) genes
Much of our foundational understanding of the cell division cycle has relied upon genetic studies using the model organism, Schizosaccharomyces pombe. To this day, S. pombe remains a powerful system in which to study this process. S. pombe: model organism S. pombe are pill shaped, single cell eukaryotes measuring roughly 3 microns wide by 10 microns long. As their moniker “fission yeast” implies, they divide through a coordinated process of cell fission. One cell divides symmetrically via a combination of cytokinesis and deposition of cell wall at the division plane. (1) The simple genetics of S. pombe -- and the general ease in which the organism can be manipulated and imaged -- makes it a powerful system in which to study a variety of cell processes. Cell-division-cycle (cdc) genes: Coordinating growth and division Regulating cell division is arguably at the intersection of the most basic requirements for life. Orchestrating this process requires the cell to consider myriad inputs – nutrient availability, nature of the environment, fidelity of the genome, etc. – to determine if, and when, to divide.(2) Many of the intricate mechanisms required for this process cannot be terminated once initiated, and the consequences of an aberrant division can ultimately determine the viability of the entire organism. The steps required to faithfully execute cell division are highly conserved across all eukaryotes, allowing insights gained using simple model organisms like S. pombe to have broad benefits to our understanding of this process. (2) The story of wee1 and cdc25 In 1975 Paul Nurse conducted a mutagenesis screen on fission yeast looking for cell-division-cycle genes. The basic study design consisted of randomly generating temperature sensitive mutants in fission yeast cells and then screening for lines that were abnormally small. Potential positives were then confirmed to be due to disruptions to the cell cycle pathway. (3) From this screen Nurse generated a mutant dubbed cdc9-50, whose small size was determined to be caused by a faster rate of division than wild type cells. Cdc9-50 was later renamed wee1, from the Scottish word for small. On the other side of this story is cdc25, which was isolated by Paul Nurse and Paul Russell using a TS screen for cell-division-cycle mutants . The mutant, identified as cdc25-22, caused cells to arrest in G2 at the restrictive temperature. (4) Attempting to elucidate cdc25’s role in cell-cycle regulation. Paul Russell and Paul Nurse first cloned the cdc25 gene in 1986 and went on to report that cells over-expressing cdc25 divided faster and were smaller than WT cells. These results supported the hypothesis that cdc25 functions to induce mitosis, but the question remained whether cdc25 and wee1 work in different pathways to regulate mitosis, or antagonistically in the same pathway. Work done by Peter Fantes demonstrated that the Cdc25-22 phenotype was suppressed in a wee1 background. Furthermore, it was discovered that the severity of the excessive division phenotype caused by over-expressing cdc25 is made worse by knocking-down wee1. Together, these findings demonstrated that cdc25 and wee1 appear to function in two distinct pathways. (5) When wee1 was cloned, its structure indicated that it likely had kinase activity, hinting at a mechanism. In 1989 Kathleen Gould and Paul Nurse demonstrated that wee1 did indeed function as a kinase and that cdc2 was a target. In vivo validation of this proposed mechanism was provided when Gould and Nurse showed that phosphorylation of cdc2 was lost as cells entered mitosis. (6) References 1. Lord M, Laves E, & Pollard TD (2005) Cytokinesis depends on the motor domains of myosin-II in fission yeast but not in budding yeast. Molecular biology of the cell 16(11):5346-5355. 2. Sveiczer A, Csikasz-Nagy A, Gyorffy B, Tyson JJ, & Novak B (2000) Modeling the fission yeast cell cycle: quantized cycle times in wee1- cdc25Delta mutant cells. Proceedings of the National Academy of Sciences of the United States of America 97(14):7865-7870. 3. Nurse P (1975) Genetic control of cell size at cell division in yeast. Nature 256(5518):547-551. 4. Russell P & Nurse P (1986) cdc25+ functions as an inducer in the mitotic control of fission yeast. Cell 45(1):145-153. 5. Fantes P (1979) Epistatic gene interactions in the control of division in fission yeast. Nature 279(5712):428-430. 6. Gould KL & Nurse P (1989) Tyrosine phosphorylation of the fission yeast cdc2+ protein kinase regulates entry into mitosis. Nature 342(6245):39-45. 7. Greenwood E (2002) Two to tango. Nature Webfocus. Milestones in cell division 9.